Field devices are commonly employed in automation technology (process automation/manufacturing automation). They serve for registering and/or influencing process variables. Examples of such field devices for process automation technology are fill level measuring devices, mass flow measuring devices, pressure- and temperature-measuring devices, pH- and redox-potential-measuring devices, conductivity measuring devices, etc., which, as sensors, register the corresponding process variables, fill level, flow, e.g. flow rate, pressure, temperature, pH-value and conductivity value, respectively.
Serving as field devices for influencing process variables are actuators, e.g. valves, which control flow of a liquid in a section of pipeline, or pumps, which change fill level in a container.
A large number of such field devices are manufactured and sold by the firm, Endress+Hauser®.
Frequently, field devices are connected with superordinated units via communication systems (Profibus®, Foundation®-fieldbus, HART®, etc.). The superordinated units serve for process control, process visualization, device-management (configuration and servicing) and for plant management (asset management), using corresponding application programs.
The integration of field devices into such applications occurs via device descriptions. Device descriptions are provided by device manufacturers, in order that superordinated units can recognize and interpret the meaning of data supplied by the field devices.
Various device descriptions are known for the different fieldbus systems (HART-device-descriptions, Fieldbus Foundation device descriptions, Profibus device descriptions).
On the basis of cooperation of Fieldbus Foundation (FF), HART Communication Foundation (HCF) and Profibus Nutzerorganisation (Profibus User Organization, known under the acronym, PNO), an electronic device description (Electronic Device Description EDD) was created, which is defined in the standard, IEC 61804-2.
With a large number of EDD-based fieldbus systems (FF, HART, Profibus) installed worldwide, EDD is an important and very widely used description language for device descriptions in automation technology.
For servicing field devices, corresponding servicing programs (operating tools) are necessary, which, in superordinated units, run either on their own (Endress+Hauser FieldCare, Pactware, AMS Fisher-Rosemount, PDM Siemens) or else are integrated into control system applications (Siemens PCS7, ABB Symphony, Emerson Delta V).
For a comprehensive servicing of field devices, newly, special device descriptions, so-called DTMs (Device Type Manager), are available, which correspond to the FDT (Field Device Tool) specifications. The FDT-specifications, serving as an industry standard, were developed by the PNO in cooperation with ZVEI (Zentralverband Elektrotechnik-und Elektroindustrie (The German Electrical and Electronics Industry, a registered association)). The current FDT-Specification 1.2.1, including the Addendum for “Foundation Fieldbus” Communication, is available from ZVEI, PNO or the FDT-Group.
Many field device manufacturers already deliver corresponding DTMs for their field devices. The DTMs encapsulate all variables and functions of the pertinent field device and offer, most often, a graphical user interface for servicing the devices. Device-specific device descriptions can already be downloaded via Internet-connections from the servers of corresponding device manufacturers.
With the help of DTMs, a device- and manufacturer-spanning servicing of field devices is possible with appropriate operating programs.
As run-time environment, the DTMs require a frame application (FDT-Frame). The frame application and the corresponding DTMs permit, thus, a very comfortable access to various variables of the field devices (e.g. to device parameters, measured values, diagnostic information, status information, etc.), as well as serving for invoking special functions, which individual DTMs make available.
Frame applications and DTMs work according to the client-server-principle.
Since the field devices are serviced via DTMs, extensive function testing is necessary, in order to assure that the DTMs work faultlessly.
This function testing has also, especially, an aspect, which is critical for safety, since safety-critical settings on field devices are effected with DTMs.
One possibility for testing DTMs is offered by the test tool dtmINSPECTOR (M&M Software GmbH, St. Georgen). For this, comprehensive test scripts are written, which are executed together with the DTM to be tested. Essentially, in these tests, it is reviewed, whether the DTM meets the FDT-specifications, thus the FDT interface definitions. The correct functioning of the DTMs as regards the device functionality is, however, not checked in such case.
The test scripts for the dtmINSPECTOR are produced individually by hand. Typical test cases based on the FDT-specifications are assembled and transformed into test scripts.
CodeWrights GmbH (Karlsruhe) produces from conventional device description files (HART, FF or Profibus), with the help of a tool (DTMstudio®) for such purpose, device-specific DTMs in large numbers. For each separate DTM, a special test script is manually produced, which, besides the interface testing, also includes testing of the device functionality. This is extremely time consuming and cost-intensive.
The more parameters a field device includes, the higher the test effort becomes. Complex field devices can, today, already have up to 1000 parameters, and more.